1. Field of the Invention
The present invention relates to an exposure control apparatus for a lens-shutter type camera with a zoom lens or plural lenses which can change a focal length of a photographing lens, and can change a full-open aperture size along with a change in focal length.
2. Description of the Related Art
A prior art apparatus of this type is disclosed in Japanese Patent Disclosure (Kokai) No. 61-162032. In this patent, a correspondence between a luminance value and a shutter speed at one focal length is stored in the form of a table. When the focal length upon photographing is changed from that of the table, the luminance value is corrected in accordance with the focal length in order to take a change in full-open aperture size into consideration, so that the luminance value is shifted on the table. Thereafter, the shutter speed is determined from this table.
However, in this exposure control method, a shutter speed can be determined in only the so-called triangular aperture region where an aperture size changes as shutter blades (sectors) are opened. More specifically, exposure control can be performed only when correct exposure is obtained before a full-open state of the shutter blades. If a long exposure time is controlled when an object to be photographed is dark and correct exposure can be obtained after the full-open state of the shutter blades, a gamma characteristic of the photometric element is changed to shift the time at which the shutter flades are fully opened. If the change is done by a switch as disclosed in Japanese Utility Model Publication (Kokoku) No. 53-24277, a mechanical system is complicated.
When a flashmatic type electronic flash is provided to a camera of this type, a flash start timing of the electronic flash must be controlled in accordance with an object distance, so that the flash emits light when an aperture is opened to a predetermined aperture size. In this case, if the focal length is changed, an aperture value is changed if the aperture size remains the same. Thus, the aperture size defining a flash timing must be changed in accordance with a focal length even for an object under an identical condition. For this purpose, a flash control apparatus for a camera is disclosed in Japanese Patent Disclosure (Kokai) No. 61-269130. In this apparatus, the correspondence among an object distance, a film sensitivity, and a flash timing is stored in the form of a table, and the film sensitivity on the table is shifted in accordance with a change in focal length, so that a flash timing for all the focal lengths can be obtained using one table.
However, the table for obtaining the flash timing is separate from a table storing the correspondence between an exposure value for exposure control and a shutter-close timing. Thus, two tables are necessary, and a storage capacity of a ROM must be increased, resulting in high cost. Further, a case wherein a flash guide number is changed along with a change in focal length is not taken into consideration.